In a Fourdrinier paper machine, paper stock is fed onto a traveling endless belt that is supported and driven by rolls associated with the machine and which serves as the papermaking surface of the machine. Fourdrinier belts are commonly formed from a length of woven Fourdrinier fabric with its ends joined together in a seam to provide an endless belt. Fourdrinier fabrics of this type generally comprise a plurality of spaced longitudinal warp filaments and a plurality of spaced transverse woof or weft filaments which have been woven together on a suitable loom. It should be noted that the warp filaments of the fabric are, for purposes of this specification, defined as those which run parallel to the machine direction of papermaking and non-woven web manufacturing machines to form a continuous carrier belt; woof or weft filaments are, for purposes of this specification, defined as those which run in the cross-machine direction.
Although the weaving and fabric treatment criteria of the present invention are applicable in other areas of monofilament, polymeric fiber fabric use, the instant features will be most readily understood in respect to the use of such fabrics for imprinting purposes in web formation operations. In these operations, for example in the operation of a paper machine according to the teachings of U.S. Pat. No. 3,301,746, improved web transferability and dryer surface contact are desirable in an imprinting fabric, and the monofilament, polymeric fiber fabric used should not contribute factors to the final paper product other than those desired by the papermaker and designed into the paper product.
In referring to monofilament, polymeric fiber fabrics herein, applicant intends reference to moist web carrier fabrics woven, for example, from the polyamide fibers, vinyl fibers, acrylic fibers and polyester fibers sold under the respective trade names of "nylon," "Saran," "Orlon," "Dacron," and "Treviera." While both wrap and woof filaments in fabrics can be made up of a multiplicity of fibers, the present invention is concerned with warp and woof filaments comprised of one fiber, i.e., monofilaments.
While a number of different weaves have been proposed for Fourdrinier fabrics, two such weaves which find extensive use today are the so-called "plain" weave and the "semi-twill" (sometimes also called "long crimp") weave. In the plain weave, each weft filament passes successively under one warp filament and then over the next warp filament, whereas in the semi-twill weave each weft filament passes over two warp filaments, under the next warp filament, and then over the next two warp filaments in a repeated pattern. Of these two weaves, the semi-twill weave is the most widely used.
The imprinting fabric suggested for use in U.S. Pat. No. 3,301,746, to which the present invention has particular relevance, may be of square or diagonal weave, and can be of any specific construction including, for example, plain or semi-twill weave. A preferred imprinting fabric, according to the teachings of the aforesaid Sanford et al. patent, has about 20 to about 60 meshes per inch and is formed from filaments having a diameter of from about 0.008 to about 0.02 inches.
Paper sheets produced in accordance with the teachings of U.S. Pat. No. 3,301,746 utilizing a monofilament, polymeric fiber, semi-twill imprinting fabric exhibit properties similar in most respects to paper sheets produced utilizing a plain weave imprinting fabric having filaments of approximately the same diameter when the semi-twill fabric is installed so that its conventional "face" side is used to imprint the uncompacted paper web. This is due to the fact that the conventional face side of the semi-twill fabric, assuming the fabric has uniform knuckle heights on its web contacting side, will produce a dot-dash pattern wherein the long axis of the dash impressions is aligned parallel to the cross-machine direction and the long axis of the dot impressions is aligned parallel to the machine direction. The dash impressions result from each weft filament passing in a repeated pattern under one warp filament and then over the next two warp filaments, while the dot impressions result from each warp filament passing in a repeated pattern over one weft filament and then under the next two weft filaments on the conventional face side of the fabric.
When paper sheets imprinted by the conventional face side of a semi-twill fabric, as described above, are doctored from the drying drum, the dot-dash knuckle impressions are aligned essentially between the creping folds. The resulting creping folds are, therefore, substantially uninterrupted across the sheet's surface. Thus paper sheets produced utilizing the conventional face side of a semi-twill imprinting fabric exhibit properties substantially similar to paper sheets produced utilizing a plain weave imprinting fabric, i.e., a low-density, soft, bulky and absorbent paper sheet characterized by having uniform creping folds which extend substantially uninterrupted across the width of the sheet.
On the other hand, utilization of the "back" side of a monofilament polymeric fiber, semi-twill fabric to imprint an uncompacted paper web in accordance with the teachings of U.S. Pat. No. 3,301,746 will, assuming the fabric has uniform knuckle heights on its web contacting side, produce a dot-dash pattern wherein the long axis of the dash impressions is aligned parallel to the machine direction of the paper machine and the long axis of the dot impressions is aligned parallel to the cross-machine direction. The dash impressions result from each warp filament passing in a repeated pattern under one weft filament and then over the next two weft filaments, while the dot impressions result from each weft filament passing in a repeated pattern over one warp filament and then under the next two warp filaments on the back side of the fabric.
Paper sheets imprinted with the back side of a conventional semi-twill, monofilament, polymeric fiber fabric, unlike paper sheets imprinted with either a plain weave fabric or the conventional face side of a semi-twill fabric, exhibit a diamond-shaped pattern after creping.
Applicant has discovered that by increasing the knuckle imprint area on the back side of a conventional semi-twill, monofilament, polymeric fiber fabric in accordance with the teachings of U.S. Pat. No. 3,573,164 issued to Friedberg et al. on Mar. 30, 1971, said patent being incorporated herein by reference, unexpected improvements in paper sheet characteristics can be realized. These unexpected advantages take the form of improved cross-directional stretch, softness, surface feel and drape. The improvements become more pronounced as the knuckle imprint area on the back side of the semi-twill fabric is increased.
Although improved web transfer characteristics and improved drying of the web are realized when the web contacting knuckle surfaces of nearly any monofilament, polymeric fiber fabric are abraded in accordance with the teachings of U.S. Pat. No. 3,573,164, applicant has learned that the aforementioned improvements in cross-directional stretch, softness, surface feel and drape are realized only with respect to the back side of a semi-twill imprinting fabric, such as is described above.
In order to maximize the beneficial effects of abrading the knuckle surfaces on the back side of a semi-twill imprinting fabric, applicant has found it desirable to obtain a semi-twill fabric having uniform knuckle heights and minimum free area on its back side prior to initiating any abrading process. Uniform knuckle heights permit a greater increase in knuckle imprint area while minimizing the danger of abrading completely through any particular filament. In addition, if knuckle heights are uniform prior to initiating any abrading process, the resulting imprint pattern after abrading will be more uniformly consistent.
Because a fabric such as is utilized for imprinting purposes in U.S. Pat. No. 3,301,746 is subjected to elevated temperatures during use, it is desirable to dimensionally heat stabilize the fabric prior to subjecting it to an abrading process to increase its knuckle imprint area. If this is not done, the uniform imprinting surface produced by carefully weaving the fabric and abrading the web contacting surface of the fabric prior to use will tend to warp as the temperature of the fabric becomes elevated, thereby losing most of the benefits to be obtained by such careful pre-treatment.
A means of preparing a dimensionally heat stable, plain weave, monofilament, polymeric fiber fabric having uniform knuckle heights and minimum free area on each side of the fabric is disclosed in U.S. Pat. No. 3,473,576 issued to Amneus on Oct. 21, 1969, said patent being incorporated herein by reference. A plain weave fabric is prepared by selecting polymeric warp monofilaments having a relatively high heat-induced shrinkage potential and further selecting an initial warp monofilament spacing in the loom according to a mathematical equation disclosed in the aforementioned Amneus patent. Polymeric woof monofilaments are then selected which have a relatively low heat induced shrinkage potential, and these woof monofilaments are woven and beaten in the weaving process into a plain weave fabric having an initial caliper calculated according to yet another mathematical equation disclosed in the aforementioned Amneus patent. After the initial weaving process, the fabric knuckles are brought to uniform heights on both sides of the fabric and the minimum free area of the fabric is set by a heat shrinkage treatment which maintains the fabric in warp tension while allowing it to shrink in the woof direction. Successive heat treatments are repeated until the monofilament, polymeric fiber, plain weave fabric does not shrink further at the treating temperature, at which point it is said to be "locked-up", i.e., no further shrinkage will occur if the fabric is later subjected, in use, to elevated temperatures equivalent to the treating temperature.
It is important to note that due to the symmetry of the plain weave, uniform knuckle heights and minimum free area are achieved simultaneously on both sides of the fabric when the weaving and heat treatment processes described in the aforementioned Amneus Patent are utilized. This is not the case with a semi-twill weave fabric. If a monofilament, polymeric fiber, semi-twill fabric is subjected to a heat treatment process similar to that disclosed in the Amneus Patent, the knuckles on the conventional face side of the fabric will become coplanar before the knuckles on the back side of the fabric have reached a uniform height. Thus, in order for the knuckles on the back side of the fabric to become coplanar, the fabric must be subjected to further heat treatment. The additional heat treatment required to make the knuckle heights on the back side of the fabric uniform causes the knuckle heights on the conventional face side of the fabric to again become non-uniform.
Therefore, the initial warp filament spacing and caliper of a semi-twill fabric necessary to produce minimum free area and uniform knuckle heights on the back side of the fabric after heat treatment are determined experimentally by trial and error.